Re: Netfilter hook doesn't see all packets

[Gordon Fisher <gordfisherman@gmail.com>]

Looks like your problem is that your iptables LOG rule tests for sport 
5353 while your hook code tests for dport 5353, which would explain why 
you're seeing different results.

 > Adding this log rule logs all packets: iptables -t mangle -I
 > PREROUTING 1 -j LOG --log-prefix="mylog" --log-level 4 --ipv4
 > -p udp --sport 5353

 >     if (dport == 5353)
 >         pr_err("sip: %pI4h, sport: %u; dip: %pI4h, dport: %u\n",
 > &sip, sport, &dip, dport);

-- 
gordonfish


On 11/20/2019 10:01 AM, Psyspy rambo wrote:
> Note: I am seeing this issue only on a specific host. It works fine on
> another host running in router mode. Any ideas to debug this?
>
> Adding this log rule logs all packets: iptables -t mangle -I
> PREROUTING 1 -j LOG --log-prefix="mylog" --log-level 4 --ipv4 -p udp
> --sport 5353
> The kernel module doesn't see ALL multicast dns packets. I assume
> iptables uses netfilter hooks too, which makes this issue strange.
> Here is the module code:
>
> static uint32_t myhook(uint32_t hooknum, struct sk_buff *skb, const
> struct net_device *in,  const struct net_device *out, int (*okfn)
> (struct sk_buff *))
> {
>      struct iphdr *ip_header;
>      uint8_t proto;
>      struct udphdr *udp_header;
>      unsigned int sip, dip, sport = 0, dport = 0;
>
>      if(!skb)
>          return NF_ACCEPT;
>
>      if(ntohs(skb->protocol) != ETH_P_IP)
>          return NF_ACCEPT;
>
>      ip_header = (struct iphdr *)skb_network_header(skb);
>      proto = ip_header->protocol;
>
>      if (proto != IPPROTO_UDP)
>          return NF_ACCEPT;
>
>      udp_header = (struct udphdr *)skb_transport_header(skb);
>      sip = (unsigned int)ntohl(ip_header->saddr);
>      dip = (unsigned int)ntohl(ip_header->daddr);
>      sport = (unsigned int)ntohs(udp_header->source);
>      dport = (unsigned int)ntohs(udp_header->dest);
>      if (dport == 5353)
>          pr_err("sip: %pI4h, sport: %u; dip: %pI4h, dport: %u\n", &sip,
> sport, &dip, dport);
>      return NF_ACCEPT;
> }
>
> /*
> pre_routing_hook_ops.hooknum = NF_INET_PRE_ROUTING;
> pre_routing_hook_ops.pf = PF_INET;
> pre_routing_hook_ops.priority = NF_IP_PRI_FIRST;
> pre_routing_hook_ops.hook = (nf_hookfn *) myhook;
> */
>
> On Thu, Nov 14, 2019 at 1:23 PM Psyspy rambo <psyspy2020@gmail.com> wrote:
>> Hello,
>>
>> I implemented a kernel module that hooks into netfilter PREROUTING
>> hook and tries to log multicast dns packet tuple. If I add a iptables
>> log rule for mdns (port 5353), it logs all mdns packets. Verified that
>> it matches tcpdump output. However, the netfilter hook sees only a few
>> packets. Any ideas why? Thanks in advance.